Commercial catalytic processes for the production of polyolefins, such as polyethylene and polypropylene, have traditionally relied on the use of heterogeneous, Ziegler-Natta catalyst systems. Typical catalyst systems for polyethylene are exemplified by Chromium supported on silica and Titanium on MgCl2. Although the catalyst systems are quite active and can produce high molecular weight polymers, they tend to produce a broad molecular weight distribution of a particular polyolefin and are poor at incorporating alpha olefins such as 1-hexene and 1-octene. When making copolymers, these catalysts typically produce polyethylene resins of moderately broad to very broad molecular weight distribution, as characterized by their molecular weight distribution of greater than 6. Lack of a narrow molecular weight in such catalyst systems is believed due to the presence of more than one type of catalytic site.
More recently, olefin polymerization catalyst systems containing well defined single reactive sites have been developed. Single-site catalysts allow for the production of polymers with varied molecular weights, narrow molecular weight distributions and the ability to incorporate large amounts of comonomers. Metallocene catalysts based on Group 4 metals of the Periodic Table (IUPAC nomenclature) containing cyclopentadienyl groups are examples of these active single-site catalysts. Such catalysts have been disclosed in U.S. Pat. Nos. 5,064,802; 5,198,401 and 5,324,800.
The mechanism of olefin polymerization has been the subject of much study and is believed to involve generation of an unsaturated, electron deficient metal species, which coordinates olefins to form intermediate alkyl olefin complexes, then subsequently undergoes rapid alkyl migration to afford a growing polymer chain. Olefin coordination followed by migration (insertion) continues until a termination step occurs or the reaction is stopped.
Several methods are currently employed to generate and stabilize the unsaturated electron deficient metal catalysts of such systems. The activation of transition metal complexes to afford stabilized, unsaturated transition metal catalysts for the polymerization of olefins is a key part of this mechanism. Several methods are currently employed to generate and stabilize the unsaturated, electron deficient metal catalysts of such systems and include halide abstraction, protonation followed by reductive elimination of an alkane or hydrogen, or oxidation. A key element of the activation process is the stabilization of the resulting activated complex using non-coordinating anions. For example, halide containing metallocene complexes can be activated using an organoalumoxane such as methylalumoxane (MAO) or isopropylalumoxane. MAO serves as both a methyl alkylating agent and a non-coordinating anion. Other activating components of utility containing boron include silver tetraphenyl borate, triphenylcarbenium tetrakis(pentafluorophenyl) borate, triaryl carbenium tetraarylborates, tris(pentafluorophenyl) boron, N,N-dimethylanilinium tetra(pentafluorophenyl) borate and sodium tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate. Catalyst systems using such activators have been disclosed in U.S. Pat. Nos. 4,808,561; 4,897,455; 4,921,825; 5,191052; 5,198,401; 5,387,568; 5,455,214; 5,461,017; 5,362,824; 5,498,582; 5,561,092; 5,861,352 and publications WO 91/09882; EP0206794B1; EP0507876B1; WO 95/15815; WO 95/23816; EP0563917B1; EP0633272A1; EP0633272B1; EP0675907B1; JP96-113779; EP0677907B1; WO 98/55518; WO 00/04059.
The greatest utility of single-site catalyst systems to the polyolefin industry is realized when they are used in existing gas phase and slurry phase reactors. Inorganic materials such as silica, alumina and magnesium chloride currently have the greatest utility as support materials in the formulation of supported Ziegler-Natta polyolefin catalyst systems. The inorganic supports have also been used with varying degrees of success in supporting metallocene and other types of single-site metal catalysts. A significant limitation of such supports, however, is the presence of surface hydroxyl groups, which render the metallocene catalysts inactive. Large quantities of MAO are used to overcome this effect, with varying degrees of success coupled with the high costs associated using MAO as a support. Polymeric supports, such as cross-linked polystyrene (PS) have been investigated as supports, since they contain no catalyst deactivating or “poisoning” groups. Methods to chemically anchor metallocene and other single-site metal catalysts have also been developed. The most common methods involve tethering the metallocene catalyst through a substituent on the cyclopentadienyl ring, through the boron atom of non-coordinating borate activators, through the nitrogen atom of an ammonium borate salt, through a substituent on the bridge of ansa-metallocene catalysts or through the heteroatom in monocyclopentadienyl complexes. The synthesis of these types of tethered complexes is difficult and generally involves multi-step, costly synthetic procedures. Thus, a general, simple process for the production of polyolefin catalyst systems that provide uniform dispersal of the catalyst, and stabilize the catalyst would, therefore, be of great utility, global economic advantage and strategic value to the commercial manufacture of polyolefins.